Presently, lasers are a common tool for use in many types of medical surgery, such as for eye surgery and the like. To obtain the desired optical power for medical surgery, Nd:YAG and other similar solid-state laser have been used. However, because of the inefficient operation of these solid-state lasers, particularly when pumped by means of flashlamps, such laser systems require water cooling as well as a large power supply. As a result, these lasers are generally not portable and require special wall plug outlets for operation. In addition, these solid-state lasers are also expensive because of their cost and complexity.
Semiconductor lasers, because of their small size, lower cost, and lower power requirements, are very attractive substitutes for Nd:YAG and other similar solid-state lasers. However, a serious problem with semiconductor lasers is that they deliver only relatively low optical power as compared with the solid-state lasers. For example, a broad area semiconductor laser with a 200 micrometer aperture will deliver only about 1 to 2 watts of output power. Therefore, as shown in U.S. Pat. No. 3,590,248 (Chatterton, Jr.), issued Jun. 29, 1971, and entitled "Laser Arrays" a conventional way to achieve more power from semiconductor lasers is to use an array of a plurality of the semiconductor lasers and combine light beams from all of the semiconductor lasers into a single beam through a plurality of optical fibers. Although an array of such semiconductor lasers can deliver as much as several tens of watts, the beam emitted from the semiconductor laser array is not easy to manage. Thus, the beam from the semiconductor laser array is very difficult to focus into a small spot so that it can be coupled into a fiber delivery system, a standard requirement for contact or non-contact surgery.
Means of obtaining high power (several tens of watts) from semiconductor lasers have been a subject of research in the past few years. In an attempt to solve the above problem, it has been suggested that one can work with a highpower semiconductor laser array and develop special fibers with a certain shape to couple as much light as possible from the array. See, for example U.S. Pat. Nos. 4,688,884 (Scifres), issued Aug. 25, 1987, and 4,763,975 (Scifres), issued Aug. 25, 1988. Spectra Diode Labs (SDL) uses a proprietary fiber that, purportedly, can couple about 10 watts from an array into a 400 micrometer core fiber with a 0.4 numerical aperture (NA). The SDL approach, although compact, has major drawbacks. At 10 watts the power is too low for most surgical application, and the NA=0.4 is too high to effectively couple the 10 watts of power into the fiber delivery system which has a standard NA of 0.37. In addition, because a large amount of heat is generated in a small area, water cooling is generally required. The SDL system is therefore not useful in some medical applications.
A different approach has been taken by Laser Diode Inc. Its LDP 4000 delivers 10 watts from a fiber bundle of 2 mm. The LDP 4000 uses 64 semiconductor lasers divided into 16 groups. Each semiconductor laser is coupled to a fiber and therefore 64 fibers are involved in the bundle. Since the standard medical fiber delivery system uses a fiber with a core diameter of equal to or less than 600 micrometers, such system apparently is not useful for medical applications. Furthermore, the system appears to have poor coupling efficiency for each individual semiconductor laser because of the large number of lasers used in the array. As a result, the LDP 4000 is not only inefficient but also is not suitable for some surgical applications.
It is also important to point out that the semiconductor lasers used by both SDL and LDI are all semiconductor lasers with emission wavelengths around 800 nm. Also, the laser arrays mentioned above are vulnerable to failure of the entire system when a single semiconductor laser element fails by exhibiting low resistance.
Efficient coupling of light from a semiconductor laser into an optical fiber has been a subject of interest for many years. This is because of the fact that the emission angles from a semiconductor diode, a PN junction, laser are not symmetrical in both the vertical (perpendicular to the PN junction of the diode laser) and horizontal (parallel to the PN junction) directions. The vertical angle of emission is generally quite large (approximately 40 degrees FWHM) compared with the horizontal angle of emission (approximately 8 degrees FWHM). This makes the coupling efficiency to a typical fiber (with a NA approximately 0.2) quite small. Efforts to increase coupling efficiency include changing the waveguide structure of the semiconductor laser, using external optics and shaping the fiber tip. Changing the waveguide structure can improve the emission angle somewhat, but always involves compromises of other operating characteristics. Using external optics involves the use of very high NA (greater than 0.5) collecting lenses (collimating lenses), and then refocusing the light into the fiber by the use of another lens (See "Ball Lenses Collimate, etc" NASA Tech Briefs, April, 1993, pp. 26-27, U.S. Pat. No. 4,995,687 (Nagal et al.), issued Feb. 26, 1991, and U.S. Pat. No. 4,186,995 (Schumacher), issued Feb. 5, 1980).
High coupling efficiency can be achieved by the use of lenses with small emitting aperture lasers, such as the lower power semiconductor lasers used in the communication, compact disc players and laser printers. However, such lenses are not useful for high power lasers with wide emitting aperture because the high NA of the lenses tends to focus the light in the horizontal direction, causing such to enter into the fiber with a larger angle than the acceptance angle of the fiber. Shaping the fiber tip has the same effect as the use of external optics but does not require sophisticated mounting mechanism for the lenses. Therefore, some other method must be derived to increase the coupling efficiency of high power broad area semiconductor lasers into optical fibers.
As is well known, all the high power semiconductor lasers comprise a structure with a wide aperture (or stripe). This wide aperture can be formed by one stripe or consist of many small stripes filling such aperture. In either case, the transverse mode in the direction of the stripes is multimode in nature and the nearer field (light distribution of the emitting facet of the semiconductor laser) is filamentary in nature (many bright spots that lase simultaneously). The light emission pattern (far field) is generally quite complicated and dependent on the input current. Direct use of such beam patterns for imaging or focusing (without using an optical lens imaging system) is not desirable. Furthermore, such broad semiconductor lasers also exhibit large astigmatism making the use of a conventional optical lens impractical. As a result, it is desirable to alter the beam characteristics before the introduction of an optics.